Lieutenant Horst Leibniz was a telepath who served in Spacefleet in the 24th century, secretly working for the Earth Security Council.

He was assigned to the Phoenix, captained by Darius Cheynor, in 2387 on its mission to combat the Phractons who'd invaded Gadrell Major. While Cheynor understood it was a suicide-mission to keep the Phracton Swarm away from Earth's solar system, Leibniz was assigned to try and make contact with the Swarm's shared intelligence network.

Leibniz worked with the Seventh Doctor and Bernice Summerfield when the Sensopath Shanstra threatened the universe. After the threat was defeated and a truce was called with the Phractons, the Doctor commented that at least Leibniz was allowed to live. He was uncertain what the Doctor meant until Phracton rebels, not trusting Cheynor, planted a bomb that killed him and Suzi Palsson.

After Cheynor's death, he was promoted to lieutenant-commander and became acting captain of the Phoenix. (PROSE: Infinite Requiem)