I’m sure folks out there have noted many of the connections between the original Inspector Morse series, the sequel Inspector Lewis, and the prequel, Endeavour, but in all my ramblings through the British detective fandom - I’ve yet to find a complete masterlist. This is my attempt at that list, but I’d welcome suggestions and contributions, and so forth - as well as any reblogs you may feel inclined to offer! I’ll credit you with any suggestions made (yes, dear reader - this is your shot at immortality.)

Morse background which has not yet appeared in Endeavour:

• Morse’s friend Roland “Roly” Marshall, who appears in the Morse episode Deceived by Flight. 
  
• Desmond McNutt, Morse’s mentor who is introduced in Masonic Mysteries must appear at some point, as will Hugo de Vries, the villain in the same episode. Here’s what we know: McNutt was Morse’s boss when he was a sergeant, as of Endeavour 3.3 he’s still a constable. 
  
• Hugo de Vries faced off with Morse twelve years before the events in Masonic Mysteries (1990). Assuming the episodes take place in the same timeline they air, Morse and McNutt will encounter de Vries in 1978.
• Possibly Elizabeth Beckett, “a sad but weII-endowed young Iady from Hinksey” mentioned in Masonic Mysteries encounters Morse at this point as well. Morse apparently became friends with de Vries’s mother later.
• In Masonic Mysteries, McNutt mentions Simon Allbright, a serial killer who Morse put away: “BriIIiant psychopath, which is what we’d be looking for. AIpha-pIus mind, gamma-minus morals. Poisoned six colleagues at work for no good reason, except that it amused him, but horrified by blood. Fainted when he cut himself shaving.”
• In Morse episode Second Time Around, Morse’s first police conference is mentioned. He mentions that he knew Patrick Dawson (played by Kenneth Colley) when they were detective sergeants underneath a chief inspector named Hillian, who then went to London, taking Dawson with him. Thanks to @oldshrewsburyian for this tip.
• In the Morse episode Promised Land Morse tells Lewis about a bank raid that went wrong, resulting in the death of his friend Ron Pigot (”Ron Pigot and l, we started in the force together.”)
  
• In the Morse episode Absolute Conviction (1992), Morse meets a prisoner named Charlie Bennett. Bennett says that when he was first arrested, Morse gave him a cup of tea. Since Bennett had been in prison for 16 years, it would have been 1976 at the latest, but probably a few years earlier, as the investigation and trial took place. 
• In 1985, Morse will run into Russell Clarke (Death of the Self), a con man who goaded a woman into suicide.

Existing connections to Morse and Lewis in Endeavour:

Characters that appear in both: 

• Dr. Max DeBryn (regular on Morse, Endeavour)
• Jim Strange (regular on Morse, Endeavour)
• Alexander Reece (Pilot, The Last Enemy)
• Susan Fallon (Pilot, Dead on Time)
• Joyce Garrett, née Morse (Home, Cherubim and Seraphim)
• Gwen Morse (Home, Cherubim and Seraphim)
  
• Matthew Copley-Barnes (Trove, The Infernal Serpent)
• Anthony Donn (Ride, Deceived by Flight)
  
• Randall Rees (Ride, Greeks Bearing Gifts)
• Marion Brooke (Arcadia, Masonic Mysteries)
• Philip Hathaway (Prey; Lewis Series 9)
• Jerome Hogg (Coda, Greeks Bearing Gifts)

PILOT:

• As Morse and the other officers from Carshall Newtown are being bussed in to Oxford at the beginning of the Pilot, “In Paradisum” from Faure’s Requiem plays, the same piece of music used which plays at an important moment in The Remorseful Day. It’s also frequently mentioned in the books as one of Morse’s favorite pieces of music.
  
• There have been plenty of mentions of Susan Spencer Fallon, Morse’s erstwhile fiance who is a major character in the Inspector Morse episode Dead on Time. She even appears in the British cut of the Endeavour pilot. (Cleverly, the show notes that she used both the names Wendy and Susan - clearing up a contradiction with the book canon - in the book The Riddle of the Third Mile she was called Wendy Spencer, in the series Susan Fallon.)
  
• The red Jaguar Mark 2 car with the registration 248 RPA is shown prominently at a car showroom, attracting interest from Morse; this is the car Morse drove in the original series.

SEASON 1

GIRL:

• Fred Thursday, while interviewing Derek Clark, says: “I know you’ve already talked to D.I. Hillian from robbery.” This is a reference to Charlie Hillian, who appears in the Morse episode Second Time Around (noticed this thanks to Christopher Sullivan’s blog post).
  

FUGUE - None.

ROCKET - None.

HOME:

• We meet Morse’s family: his father, Cyril Morse, his stepmother, Gwen, and his half-sister Joyce. Joyce and Gwen appear again in the Morse episode Cherubim and Seraphim.
• Morse’s injury in this episode is supposed to cause the slight limp which John Thaw exhibited.

SEASON 2

TROVE:

• This episode mentions Danny Griffon, who is the grandfather to his namesake in the pilot episode of Lewis. Source.
• Oxford Don Matthew Copley-Barnes appears, and references are made to his daughter Imogen and her friend Sylvie. These characters appear in the Morse episode The Infernal Serpent.
  
• The “Wolvercote Trove” is important to the plot. This references the Morse episode The Wolvercote Tongue. 
• This is the first mention (I think) of the Masons, by Jim Strange (Morse: Masonic Mysteries). 

NOCTURNE:

• Bloody Charlotte is confined to Holmwood Park Sanatorium. This is a fictional location also used in Lewis episode Falling Darkness (which also has a character going by the alias “Charlotte.”)
  

SWAY - None.

NEVERLAND:

• This episode mentions Blenheim, which is important in the Inspector Lewis episode Fearful Symmetry (source).
• Thursday references Desmond McNutt, who appears in Morse episode Masonic Mysteries.

SEASON 3

RIDE:

• Anthony “Tony” Donn is a character in this episode of Endeavour. He is an old school friend of Morse’s who also appears in the Morse episode Deceived by Flight.
  
• Kay Belborough refers to Guy Mortmaigne, a relative of the Mortmaignes who appear in the Lewis episode Falling Darkness and Julius Hanbury, who appears in the Morse episode Ghost in the Machine.
• Randall Rees is a minor character - he’ll later appear in the Morse episode Greeks Bearing Gifts.
• Morse explains the origin of his school nickname, Pagan. This was also discussed in Deceived by Flight. 

ARCADIA:

• Marion Brooke appears in this episode as a member of AMNOX. She’ll later feature in the Morse episode Masonic Mysteries. According to that episode, she worked at the university and was the one Hugo De Vries “conned the most.” Morse will run into her again in the 70s.
  

PREY:

• Philip Hathaway appears in this episode of Endeavour. He is James Hathaway’s father, and also appears in the Inspector Lewis episodes One For Sorrow, Magnum Opus, and What Lies Tangled.

CODA:

• Peter Matthews, Tommy Thompson, and Bernie Waters, who are mentioned in the Morse episode Promised Land, appear.
  
• Jerome Hogg - who was also in the Morse episode Greeks Bearing Gifts - appears.

SEASON 4

GAME - None.

CANTICLE - None.

LAZARETTO:

• Caroline Bryce-Morgan is a major character in this episode. She mentions her son William Bryce-Morgan and daughter Susan Fallon. Both William and Susan appeared in the Morse episode Dead on Time. 
  

HARVEST - None.

SEASON 5

MUSE - None.

CARTOUCHE - None.

PASSENGER - Appearance of a young Patrick Dawson, who will later meet Morse in Second Time Around. 

Connections to Morse in Lewis:

SEASON 1

PILOT:

• In the pilot episode of Lewis, there’s plenty of talk about Morse and a cameo from The Car. 
• Danny Griffon appears. His grandfather is mentioned in the Endeavour episode Trove.

Season 9

• Philip Hathaway appears in Lewis episodes One For Sorrow, Magnum Opus, and What Lies Tangled. He also appears in the Endeavour episode Prey.
• In the final episode of Lewis, Lewis says he wants to continue working - “like Morse.”

I’ll be honest, I’m not as familiar with Lewis, so any suggestions are welcome