Crusade is a spin-off

Spin-off (media)

In media, a spin-off is a radio program, television program, video game, or any narrative work, derived from one or more already existing works, that focuses, in particular, in more detail on one aspect of that original work...

 TV show from J. Michael Straczynski

J. Michael Straczynski

Joseph Michael Straczynski , known professionally as J. Michael Straczynski and informally as Joe Straczynski or JMS, is an American writer and television producer. He works in films, television series, novels, short stories, comic books, and radio dramas. He is a playwright, a former journalist,...

's Babylon 5

Babylon 5

Babylon 5 is an American science fiction television series created, produced and largely written by J. Michael Straczynski. The show centers on a space station named Babylon 5: a focal point for politics, diplomacy, and conflict during the years 2257–2262...

. Its plot is set in AD 2267, five years after the events of Babylon 5, and just after the movie A Call to Arms

Babylon 5: A Call to Arms

A Call To Arms is the fourth feature-length film set in the Babylon 5 universe . It was written by J. Michael Straczynski, directed by Mike Vejar, and originally aired January 3, 1999.-Summary:...

. A race called the Drakh

Drakh

The Drakh are a fictional alien race in the Babylon 5 universe. The Drakh were dark servants for the race of First Ones known as the Shadows. They appeared in the fourth and fifth seasons of Babylon 5 as well as the television movie, A Call to Arms...

 have released a nanovirus plague

Grey goo

Grey goo is a hypothetical end-of-the-world scenario involving molecular nanotechnology in which out-of-control self-replicating robots consume all matter on Earth while building more of themselves, a scenario known as ecophagy .Self-replicating machines of the macroscopic variety were originally...

 on Earth, which will destroy all life on Earth within five years if it is not stopped. To that end, the destroyer Excalibur

Excalibur (Crusade)

The IAS Excalibur is a starship in the fictional Babylon 5 / Crusade universe. First seen in the Babylon 5 movie A Call to Arms, the ship is a Victory class destroyer. The ship combines human, Minbari, and Vorlon technology....

has been sent out to look for anything that could help the search for a cure.
Like Babylon 5, Crusade was intended to have a five-year story arc

Story arc

A story arc is an extended or continuing storyline in episodic storytelling media such as television, comic books, comic strips, boardgames, video games, and in some cases, films. On a television program, for example, the story would unfold over many episodes. In television, the use of the story...

, although as Straczynski notes in the DVD commentary for A Call To Arms, it was intended to resolve the Drakh plague after a season or two and move onto other arcs.