7x21 spoilers:
"Life Sentence" continues a storyline started in the previous episode, "Nothing for Something". Mac's former partner William Hunt (Peter Fonda) came back, and a criminal named Raymond Harris (Clifton Collins, Jr) got out of jail. Harris has a vendetta against Hunt, and he attacks the crime lab at the start of "Life Sentence". Mac and Danny go to Harris's old apartment with a warrant. They knock on the door and identify themselves, but they get no answer. They break down the door and rush into the apartment. Bullets start flying through the closed bedroom door, so Mac and Danny return fire. They each empty a clip from their gun, and there is silence when they stop to reload. They look into the bedroom, and they see a dead man on the ground with four bullet wounds in his chest and a gun on the ground next to him-but it's not Raymond Harris.
Internal Affairs investigates the shooting, and Mac and Danny take turns speaking with Detective Angela Sayer. She takes their guns, and she asks them why they entered the apartment without waiting for ESU. Mac says Harris is a flight risk, but the fact remains that Harris isn't in the apartment, and they have an unidentified man lying dead on the bedroom floor.
While Mac and Danny were firing their guns in Harris's apartment, Harris was at the precinct filing a complaint against William Hunt. Internal Affairs closes the case on Danny and Mac, clearing them and returning their weapons. Detective Sayer tries to ask Mac questions about what's going on with Raymond Harris and William Hunt, but Mac won't elaborate on the situation. Detective Sayer advises Mac to stay away from Harris, but Mac tells Flack to make sure the police keep an eye on the man.
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