The Truth About Love (2005) on Netflix

Plot summary

Alice Holbrook lives in Bristol with her husband Sam, a lawyer, whose best friend and law partner is Archie Gray. An unsigned Valentine's card with radish seeds tucked inside arrives for Alice, and she takes it at once for a romantic gesture from Sam. She writes an anonymous reply, and after a drunken evening with her sister Felicity the card goes to Sam; the sisters had made a bet about whether he would hide it. He hides it. What began as a prank leads to events and revelations that put the marriage at risk, and Alice is left looking for answers.