Quantum theory is strange because it lets objects do strange things, like be in two places at once, but relativity is stranger because it alters space and time, which is almost unthinkable. If an electron is a point on a screen, quantum theory describes how the point changes, but relativity describes how the screen itself changes in unexpected ways.
QR5.2.1. Our Reality Bubble
QR5.2.2. Invariance
QR5.2.3. Maintaining Causality
QR5.2.4. The Ultimate Messenger
QR5.2.5. The Universal Speed Limit