The Year of the Slow Invasion

The Invasion of the Cubes, otherwise known as the Shakri invasion of Earth, was an attempted genocide by the Shakri on 21st century Earth. They studied life on Earth by placing monitoring cubes all over the planet, which humans quickly became accustomed to. By taking these cubes into their homes, and normalising their presence by posting about them on social media, humanity became the instrument for their own destruction.

After a year observing humanity, the Shakri determined that the heart was a major weakness in human biology, and attempted to kill one third of the population with cardiac arrests, brought on by the cubes. They were stopped by the Eleventh Doctor, who repurposed the cubes to restart victims' hearts, aided by UNIT. (TV: The Power of Three)