Anthony Blunt's exact entry into Soviet intelligence is disputed, but it always ties back to one thing: his relationship with Burgess. One of them recruited the other, although accounts vary as to which way it went. The two men met at Cambridge and became close friends; both were known to be gay, and they may have had a romantic relationship as well. During World War II, Blunt worked for the Security Service, MI5, and passed intelligence from decrypted German broadcasts to the Soviets. That intelligence, from Operation Ultra, was intensely secretive and sensitive.
When Burgess and MacLean defected, Blunt was distraught. His own actions were not revealed until 1963, when an American he'd recruited turned him in. He confessed in 1964 and also named other spies, including John Cairncross, the Bletchley Park operative he'd brought in for the Ultra mission. In return for this full confession, he was granted immunity, and his spying was kept secret for 15 years. In 1979, Margaret Thatcher revealed Blunt's spying; his career ended immediately, and he died in 1983.