Allen Morgan, 73, and Margaret Morgan, 75, are accused of conspiring together to kill Carol Morgan.
The 36-year-old was found battered to death in the shop she ran with her husband in Linslade, Bedfordshire, in August 1981.
Allen and Margaret Morgan had been in a “passionate, but forbidden and adulterous love affair” at the time, it was said.
Jurors were told the couple, who are now married, cooked up a plan with an unknown killer to get Carol out of their way.
Prosecutor Pavlos Panayi KC said: “The murder of Carol Morgan was no random attack. It was planned and paid for by the two defendants in the dock.
“Had she not been murdered and had she lived these past 43 years, she would be 80 years old.
“She was killed on the evening of 13 August 1981 in a brutal attack by someone who has never been identified.
“That someone used an axe or heavy knife or machete to hack into her body and skull, causing horrifying injuries from which she died.”
Luton Crown Court was told Allen Morgan embarked on the affair with Margaret a year before the killing.
He couldn’t get a divorce from Carol because she brought the money into the marriage and there were “spiralling debts”, it was said.
On the day she died, Allen Morgan took Carol’s two children from a previous marriage to the cinema in Luton.
The court heard this was unusual as he “didn’t have a strong relationship with his stepchildren” and had never before taken them out without their mum.
The trio had not been due back until 11pm – leaving Carol alone in the shop, jurors were told.
After Carol closed the shop, an attacker or attackers killed her inside the store room at around 7pm.
Allen Morgan claimed he returned home to find his wife dead and said £400 cash had been taken from a desk and £35 from the till, along with 1,400 cigarettes.
Police found “no sign of any forced entry” and the family dog had been shut into a bedroom, it was said.
The widower was quickly ruled out as the murderer because of his alibi and he denied playing any part in the horror.
After his wife was killed, Margaret Morgan left her husband and moved in with Allen Morgan.
The couple, who deny conspiracy to murder, have been married ever since.
The trial continues.