Dead Cert(1974)

As a surprise, two horse owners decide to ride their animals themselves in a steeplechase. But Bill Davidson's horse "Admiral" behaves weirdly, and falls hard after an obstacle. Bill dies from his injuries. His friend Alan York 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 exact same fate as his friend.