K-pop superstars BTS returned to the stage Saturday night with a free comeback concert in central Seoul, marking their first full-group performance in nearly four years.

You’re not watching a scene from the latest28 Years Later movie. This is real-world K-pop star Park Sung-hoon, running for his life.

The concert in Seoul’s historic center was a grand homecoming for a pivotal force of South Korean soft power. RM, the group’s leader, performed through an ankle injury.

Local reports estimate that roughly 40,000 people gathered across central Seoul to watch K-pop band BTS reunite.

BTS has returned as a full group with a huge, free concert in Seoul after a four-year break. The K-pop band played new songs ...