Y Sŵn is a Welsh language period film which follows the events of 1979-80 leading up to the creation of Welsh medium television channel S4C. The film dramatises the fallout of the announcement made by Home Office, led by home secretary Willie Whitelaw, to renege on a Conservative election campaign pledge to establish a Welsh language channel. The film focuses on Plaid Cymru president Gwynfor Evans's campaign to hunger strike in protest at the government's u-turn, alongside the efforts of others to secure the channel, including campaign group Cymdeithas yr Iaith and Plaid Cymru MPs Dafydd Wigley and Dafydd Elis-Thomas.

Sian Reese-Williams appears as Margaret Thatcher, with Mark Lewis Jones as Willie Whitelaw and Rhodri Evan as Gwynfor Evans.