Surah Nuh Tafseer

And they have plotted a mighty plot.

is that they plotted a deceptive plot for their followers tricking them into believing that they were following the truth and correct guidance.

This is like what they will say to them on the Day of Judgement,

بَلْ مَكْرُ الَّيْلِ وَالنَّهَارِ إِذْ تَأْمُرُونَنَأ أَن نَّكْفُرَ بِاللَّهِ وَنَجْعَلَ لَهُ أَندَاداً

Nay, but it was your plotting by night and day:when you ordered us to disbelieve in Allah and set up rivals to Him! (34:33)

For this reason He says here,

The Idols of the People of Nuh and what happened to Him

وَمَكَرُوا مَكْرًا كُبَّارًا

وَقَالُوا لَا تَذَرُنَّ الِهَتَكُمْ وَلَا تَذَرُنَّ وَدًّا وَلَا سُوَاعًا وَلَا يَغُوثَ وَيَعُوقَ وَنَسْرًا