Complex sentence: Due to a malfunction of the TARDIS' "chameleon circuit", which normally allows the TARDIS to take on the appearance of local objects to disguise it from others, the Doctor's TARDIS remains fixed as a blue British Police box.
Split into simpler sentences: Due to a malfunction of the TARDIS' "chameleon circuit", which normally allows the TARDIS to take on the appearance of local objects to disguise it from others. However, the Doctor's TARDIS remains fixed as a blue British box or Police box.