Thank you for your question.
As noted on TV Tome, the last episode for Chester was episode 333:
gs: Jan Clayton (Clara Wright) Charles McGraw (Albert Calvin) June
Dayton (Emily Calvin) Bill Erwin (Ned Wright) Gene Lyons (Fletcher)
Burt Reynolds (Quint Asper)
Ned Wright is on his death bed. His wife Clara goes to town to get
Doc. Mrs. Crown tells Matt she doesn't want a murderer's wife in town.
The Calvins watch the angry mob around Clara. Ned was tried for
killing David Bently but was acquitted. Ned admits to killing Bently
to Doc and Clara, then dies. Emily Calvin asks her husband Albert if
Clara could stay with them. Quint tells Chester he doesn't think Ned
did it. Fletcher, a gunman, comes into the Long Branch. Calvin was
Bently's best friend. Chester asks him if Bently had any enemies.
Calvin hires Fletcher to kill Chester. Fletcher agrees on $500 then
takes notice of the house and upd it to $1000. Emily asks why did you
give that man so much money. Calvin accuses Emily of fooling around
with Bently. Calvin says he killed Bently. Fletcher tells Chester he's
been hired to kill him. Fletcher says he's never killed anyone.
Chester gives him $18, but he won't tell who hired him. Chester
figures it's Calvin. Chester tells Calvin someone stole $18 from him.
Albert's going to kill Chester. He locks Emily up and leaves with a
shotgun. Emily climbs out of the window and runs to tell Chester,
Albert's gonna kill you. Chester sees Albert outside and yells duck to
Emily. Albert fires. Emily is killed. Albert returns home and can't
find Emily. Chester brings Emily's body to Albert. Clara says goodbye
to Albert. He tells her he killed Bently. Ned confessed so people
would take pity of Clara. Albert killed and Ned and Clara suffered.
b: 11-Apr-1964 w: John Kneubuhl d: Harry Harris
NOTE: The last episode to feature Chester (Dennis Weaver)..."
This page mentions that Weaver left Gunsmoke for another role:
"...Dennis Weaver left Gunsmoke to play a veterinarian in what television series?
Kentucky Jones ... it ran for one season. The series was originally
titled Kentucky's Kid and was designed to star Jack Carson..."
Australia's TV Time notes the following on his leaving in a 1962
interview with James Arness:
"...Arness said: "We've all probably had our secret doubts about going
on with it." He pushed his dusty Stetson back on his head and added
candidly: "Maybe there isn't much more I can do with the role after
all this time. But so what? The dough's stillcoming in, and I could
keep on playing Matt Dillon forever. When Dennis left the show this
past season to seek out greener pastures I told him he'd be back.
And," Arness pointed out, "he did return. But for awhile it was kind
of tough trying to figure out what would happen to the show with him
gone. Public opinion called him back. We were swamped with letters
asking for him. So when his own TV show idea fell through he came
back. I was mighty happy to see him."...
Evidently, after 7 years or so relationships had eroded among the
actors. Weaver left for the starring role mentioned above.
chester's OR weaver's +"last episode" +gunsmoke
why did dennis weaver leave gunsmoke
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