The programme, which is currently being filmed, focuses on five overlapping stories of romance and love.
The show has been penned by Dive writer Dominic Savage, but all of the instalments will be partially improvised.
Tennant will play a happily married man who begins to re-evaluate his life when his first love shows up again.
The other stories will be led by Billie Piper, Jane Horrocks, Ashley Walters and David Morrissey.
This Is England star Vicky McClure will appear in Tennant's episode, while other stars who have recently joined the show include Lacey Turner, Jaime Winstone and Jenny Agutter.
The BBC's controller of drama commissioning Ben Stephenson has previously described the partially improvised show as "a complete first in TV drama on BBC One" and added: "It features some of our very best acting talent working in a way we've never seen on screen from them before, using improvisation to push the boundaries of each story, to get right to the heart of the truth."
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