Dead Cert (1974)

As a surprise, two horse owners decide to ride their animals in a steeplechase. But Bill Davidson's (Ian Hogg's) horse "Admiral" behaves weirdly, and falls hard after an obstacle. Bill dies from his injuries. His friend Alan York (Scott Antony) suspects the animal was doped by unscrupulous bookies and starts to investigate. He doesn't know how serious his opponents are, and that he's in danger to suffer the same fate as his friend.